1. Technical Field:
This invention pertains to semiconductor packaging in general, and in particular, to a method and apparatus for making semiconductor packages with tacked dies and reinforced wire bonds.
2. Related Art:
Practitioners have responded to an increasing demand for semiconductor packages that are smaller, yet more functional, by stacking two or more semiconductor dies, or xe2x80x9cchips,xe2x80x9d in one package. FIGS. 1 and 2 show a prior art ball grid array (xe2x80x9cBGAxe2x80x9d) semiconductor package 10 having a pair of stacked dies 14 and 16. First die 14 is attached to the top surface of an insulative substrate 12 with an adhesive 13. Second die 16 is stacked on the top surface of first die 14, and is attached thereto by an adhesive layer 15. Both dies 14 and 16 have peripheral bond pads 34 that are electrically connected by conductive wires 38 to respective pads 26 in a metal layer 22 on the top surface of substrate 12. Adhesive layer 15 is positioned entirely within the perimeter of die bond pads 34, and provides clearance for the wires 38 that are connected to first die 14. The pads 26 in metal layer 22 are electrically connected to traces of a metal layer 24 on the bottom surface of substrate 12 by metallized vias 23 through substrate 12. Solder balls 25 are fused to lands 27 formed in metal layer 24. A molded plastic body 60 (shown by dotted outline in FIG. 2) encapsulates dies 14, 16, wires 38, and the top surface of substrate 12.
While this package has enhanced component density, it also has certain drawbacks. For example, during wire bonding, second die 16 may crack due to downward force applied by a wire bond head 62 to the cantilevered peripheral portion of die 16. In addition, a wedge of the molding compound may penetrate the space 19 between stacked dies 14, 16. Since the mold compound typically has a different coefficient of thermal expansion than adhesive layer 15, the two dies may crack apart. Another problem is xe2x80x9cwire sweep,xe2x80x9d i.e., the tendency of the relatively long bond wires 38 of second die 16 to be bent or broken by the rush of molten plastic during molding. Accordingly, there is a need for an improved package design and manufacturing method.
This invention provides a simple, inexpensive method for making a semiconductor package with stacked dies that eliminates fracturing of the dies during the wire bonding process or as a result of incompatible thermal expansions, and that also eliminates the problem of broken wire bonds as a result of wire sweep. The method permits the use of ultra-thin dies having either the same or different sizes.
In one embodiment, the method includes the provision of a substrate, which may be either a conventional laminate or a lead-frame-type of substrate, and a pair of semiconductor dies. Each die has opposite top and bottom surfaces and a plurality of wire bonding pads located around the periphery of the top surface thereof. The bottom surface of the first die is attached to a top surface of the substrate, and the wire bonding pads on the first die are connected to wire bonding areas on the top surface of the substrate with a first plurality of conductive wires.
A measured quantity of an uncured, electrically non-conductive, viscous fluid adhesive is dispensed onto the top surface of the first die. The bottom surface of the second die is then pressed down onto the adhesive to squeeze, or distribute, it laterally between the two dies and toward their respective peripheries until: 1) the bottom surface of the second die is spaced apart from the top surface of the first die by a layer of the adhesive thick enough to prevent shorting contact between the second die and the conductive wires bonded to the top surface of the first die; 2) the entire bottom surface of the second die, including any portion overhanging the first die and the substrate, is supported from below by the layer of adhesive; and, 3) the wire bonding pads on the top surface of the first die and the inner ends of the conductive wires bonded thereto are encapsulated by the adhesive. The adhesive layer is then cured, and the wire bonding pads on the top surface of the second die are wire bonded to wire bonding areas on the top surface of the substrate with a second plurality of conductive wires.
After the second die is wire bonded to the substrate, a bead of adhesive is dispensed around the entire periphery of the stack of dies such that the exposed portions of the wires connecting both dies to the substrate are completely encapsulated within the bead of adhesive, which is then cured to harden it. The bead of adhesive can be the same material as the layer of die attach adhesive applied between the dies, or a material with the same or a very similar coefficient of thermal expansion. The bead of adhesive contacts and adheres to the entire perimeter of the layer of hardened die attach adhesive.
The hardened layer of adhesive below the entire bottom surface of the second die supports the die from below and prevents the second die from being fractured by forces acting thereon during wire bonding. In addition, because the adhesive layer fills the peripheral space between the opposing surfaces of the two dies, the layer prevents the molding compound or other encapsulant formed over the stacked dies from flowing into the space and forming a potentially destructive xe2x80x9cthermal wedgexe2x80x9d between the dies. Further, since the adhesive layer also covers the wire bonding pads on the top surface of the first die, as well as the inner ends of the conductive wires that are bonded thereto, it helps maintain the integrity of the electrical connection between the wires and the first die during the subsequent handling and manufacturing processes. The hardened bead of adhesive around the periphery of the dies encloses the portions of the conductive wires not encapsulated by the die attach adhesive layer, including the ends of the wires bonded to the pads on the second die, and the middle portions and outer ends of the of the wires attached to both dies, thereby reinforcing the wires and preventing bending or breakage caused by wire sweep during package encapsulation.
A better understanding of the above and other features and advantages of the invention may be had from a consideration of the detailed description below of some exemplary embodiments thereof, particularly if such consideration is made in conjunction with the appended drawings.